Re: [PATCH 1/2] simplify-rtx: Simplify trunc of and of shiftrt

[Segher Boessenkool <segher@kernel.crashing.org>]

On Tue, Nov 10, 2015 at 10:04:30PM +0100, Bernd Schmidt wrote:
> On 11/10/2015 06:44 PM, Segher Boessenkool wrote:
> 
> >Yes I know.  All the rest of the code around is it like this though.
> >Do you want this written in a saner way?
> 
> I won't object to leaving it as-is for now, but in the future it would 
> be good to keep this in mind.

With the trunc_int_for_mode it ended up hugging the righthand margin,
so I did clean it up after all.  Please see attached (committed).


Segher


diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index 17568ba..c4fc42a 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -714,6 +714,34 @@ simplify_truncation (machine_mode mode, rtx op,
     return simplify_gen_binary (ASHIFT, mode,
 				XEXP (XEXP (op, 0), 0), XEXP (op, 1));
 
+  /* Likewise (truncate:QI (and:SI (lshiftrt:SI (x:SI) C) C2)) into
+     (and:QI (lshiftrt:QI (truncate:QI (x:SI)) C) C2) for suitable C
+     and C2.  */
+  if (GET_CODE (op) == AND
+      && (GET_CODE (XEXP (op, 0)) == LSHIFTRT
+	  || GET_CODE (XEXP (op, 0)) == ASHIFTRT)
+      && CONST_INT_P (XEXP (XEXP (op, 0), 1))
+      && CONST_INT_P (XEXP (op, 1)))
+    {
+      rtx op0 = (XEXP (XEXP (op, 0), 0));
+      rtx shift_op = XEXP (XEXP (op, 0), 1);
+      rtx mask_op = XEXP (op, 1);
+      unsigned HOST_WIDE_INT shift = UINTVAL (shift_op);
+      unsigned HOST_WIDE_INT mask = UINTVAL (mask_op);
+
+      if (shift < precision
+	  /* If doing this transform works for an X with all bits set,
+	     it works for any X.  */
+	  && ((GET_MODE_MASK (mode) >> shift) & mask)
+	     == ((GET_MODE_MASK (op_mode) >> shift) & mask)
+	  && (op0 = simplify_gen_unary (TRUNCATE, mode, op0, op_mode))
+	  && (op0 = simplify_gen_binary (LSHIFTRT, mode, op0, shift_op)))
+	{
+	  mask_op = GEN_INT (trunc_int_for_mode (mask, mode));
+	  return simplify_gen_binary (AND, mode, op0, mask_op);
+	}
+    }
+
   /* Recognize a word extraction from a multi-word subreg.  */
   if ((GET_CODE (op) == LSHIFTRT
        || GET_CODE (op) == ASHIFTRT)
-- 
1.9.3